The present invention relates in general to a method and apparatus for securing pressure-containing equipment, such as a pipe clamp, in remote or hazardous locations. More particularly, the invention relates to a method and apparatus for pretensioning bolts using a robotic device or mechanical manipulator.
A very common problem in the assembly of pressure-containing equipment is the proper tightening of groups or sets of bolts used to clamp sealing surfaces together. It is important to get uniform tensioning of the bolting in order to ensure pressure containment and avoidance of bolt overstress or pressure vessel distortion. This same problem occurs in some types of structural connections, such as securing mated flanges.
The most commonly used approach in manual or machine tightening of sets of bolts is the sequential tightening of the bolts with multiple cycles of incremental load application to each of the bolts by means of a torque wrench. A typical procedure of this type would be the tightening of the head bolts on an engine. Such an approach is noted for being fairly sensitive to frictional variations and is therefore prone to inaccuracies.
Pipe clamps used for repairing a localized leak in a pipeline consist essentially of a resilient gasket that is placed over the leak and held with sufficient force to effect a seal. Sealing pressure-containing equipment typically involves releasing the pressure, placing the seal in the proper location, applying pressure to the seal, and reintroducing the pressure into the pipeline or pressure-containing equipment. The clamp or other device that applies loading to the seal is pretensioned, or pretightened to apply the appropriate amount of pressure, before the pressure is reintroduced into the pressure-containing equipment.
Pipe clamps are commonly used in land-based and underwater applications which cannot be directly serviced by personnel, such service being performed by robots or by personnel working from some distance utilizing submersible craft with remote manipulators. One limitation of robots and remote manipulators is that they cannot reliably or efficiently perform fine mechanical maneuvers such as threading together a nut and a bolt. Furthermore, in remote applications it becomes difficult to provide uniform tightening of sets of bolts, where each bolt must be tightened securely with a uniform application of force to the nuts.
It has been common practice, in general construction work that uses conventional helically threaded bolts for fixedly securing mating flanges, to thermally stretch or elongate hollow bolts prior to tightening the nut with a torque wrench or other rotational means. By thermally elongating a bolt, one can uniformly tighten a bolt with minimal effort and the bolt will be appropriately pretensioned when it cools. By applying thermal stretching procedures to bolts, one can predetermine tension preload while minimizing the rotational torque required to achieve the desired tension on the bolt. Likewise, in releasing such bolts, the use of thermal elongation is also used to enable the associated nut to be loosened with less force.
The heating of bolts having an axial bore with an internal heating means such as an electrical heating element has been described, as for example in U.S. Pat. No. 4,991,563. While the use of electric heating devices permit a very reliable means of obtaining a uniform manual pretensioning of bolts, use of such heating devices to ensure the proper bolt tightening of pressure containing equipment in remote applications has not been described. In addition, no methods or apparatus has been described that would allow the simultaneous heating of a group of bolts to ensure the uniform heating of a series of bolts to allow uniform pretensioning of the bolts.
In fact, the use of such heating devices with conventional tightening procedures is neither simple nor practical in remote applications. Tightening bolts in such remote applications is difficult because of a lack of visibility of the bolts, equipment access to the bolts or nuts and equipment operational limitations such as proper alignment. U.S. Pat. No. 5,437,489 describes a method of using a hydraulically-thrusted wedge system to pretension bolts at a remotely installed pipe clamp using a wedge system. However, the means illustrated in that patent requires a very large thrust on each wedge in order to achieve a sufficient tightening of the bolting of the clamp. The large magnitude of the tightening thrust is due to the high friction inherent in the use of wedges for imposing high axial stresses. The thrust is provided by a hydraulic cylinder, but controllability of pretension is limited by the unknown frictional resistance to the pretensioning of the bolting and the size and cost of the necessary actuating cylinder and its supporting structure.
Thus, a need exists for a method and apparatus for remotely pretensioning a bolted connection which is easily operated by routine construction equipment, particularly a method and apparatus for uniformly pretensioning a series or group of bolts.
A further need exists for a method and means to simply reverse the remotely performed pretensioning of a bolted connection with minimal force.
The invention contemplates a simple, inexpensive device for solving the problems and disadvantages of the prior approaches discussed above. The present invention provides a rapid, reliable and accurate tightening of bolting in remote locations.
One aspect of the present invention is an apparatus for prestressing structural joint connections comprising: (i) two clampable elements with comating first surfaces; (ii) first and second wedging elements having comating ramps, the second wedging element positioned between the first wedging element and one of the clampable elements; (iii) one or more connecting elements traversing the two clampable elements and the first and second wedging elements such that a first end of each connecting element engages the outside surface of one of the clampable elements and the second end of the connecting element engages the outside surface of the first wedging element; and (iv) heating means for heating the connecting elements. Thus, when the ramped face of the first wedging element is moved it changes the tension on the connecting elements thereby changing the pressure between the comating surfaces of the clampable elements. The movement of the ramped face of the first wedging element is particularly effective when the heating means have been activated to heat the connecting means thereby thermally expanding the lengths of the connecting means and decreasing the force necessary to move the first wedging element.
In accordance with another aspect of the invention an apparatus is described for uniformly thermally expanding a set of bolts comprising: (i) a set of bolts, each bolt having an internal bore; (ii) a plurality of electrical heating elements, wherein one heating element is placed within the internal bore of each bolt; (iii) a power source; and (iv) a series of electrical connectors, each connector connected to the power source, to one of the heating elements within one bolt and to a switch where the switch responds to the temperature of the one bolt as determined by a temperature sensor in communication with the bolt and the switch.
In accordance with yet another aspect of the invention, the bolt shafts are preheated to a predetermined temperature above ambient, pretensioned at a first, lower level, at the increased temperature using a wedge system, and allowed to cool to ambient temperature., whereby the bolt shafts are pretensioned to a second, higher level.
In accordance with still yet another embodiment of the present invention, the prestressing of structural joint connections is accomplished by: (i) encompassing a structural joint connection with an apparatus for prestressing structural joint connections, the apparatus comprising first and second clampable elements, said clamping elements having comating first surfaces and obverse second surfaces; a first wedging element having a first face and a second obverse face, said first face having at least one ramp; a second wedging element having a ramped surface, wherein each ramp of the ramped surface comates with one ramp of the first wedging element, wherein said second wedging element is positioned between the second obverse surface of the first clamping element and the ramped first face of the first wedging element; one or more connecting elements traversing the two clampable elements, the first wedging element and the second wedging element, each connecting element having a first end engaging the second surface of the second clampable element and a second end engaging the obverse face of the first wedging element; and heating means for heating the connecting elements, wherein one heating means selectively transfers heat to each connecting element; (ii) thermally expanding the connecting elements to obtain increased lengths of said connecting elements relative to the unheated lengths of the connecting elements thereby reducing the tension on the connecting elements; (c) activating a double-acting hydraulic means connected to the first wedging element; (d) moving the first ramped surface of the first wedging element to produce an initial wedging of the comating ramped faces of the first wedging element and the second wedging element to achieve an increased level of tension on the connecting elements; and (e) cooling the connecting elements to decrease the length of the connecting elements to further increase the tension on the connecting elements thereby increasing the compression on the comating first surfaces of the two clampable elements.
The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.